Great Pyrenees Chasing Semis

I have a one year old, male GP. He is an excellent dog except for the fact that he has been chasing semi trucks that drive by our house. I realize I could pen him up to stop it, but that would make it hard for him to protect my free ranging flock. I have caught him doing it twice, but I know he does it during the day when I'm not home. Each time I caught him doing it, I really scolded him and penned him up for a day or two. That has not worked.

sorry, but he needs to be fenced. These dogs are known to roam and chase predators. Perhaps he thinks the truck is after his chickens. At any rate, scolding him won't do any good. He needs to be fenced for his own protection. And for yours too. Think about what would happen if he caused an accident because of running into the road. They would come after you. So fence that dog to protect him and yourself.

Like Carolinagirl is saying; he need to be either fenced, or put on a long runner, or something...
That's the first one I heard; dog chasing semis!
And that is one dangerous behavior. Sorry that this happened to your dog.
Maybe he forgets about this when he gets older and when he'll be confined for some time.
I really don't know what else to tell you.

Thanks.
If penning him up is the only way, I may as well find him a new home. I can't stand the idea of penning a dog up, especially when I have over 5 acres for him to run, and he won't be able to guard the flock.

fence along the road. Put up an electric fence and boundary train him to the 5 acres. There are many ways to handle a dog chasing cars. It will require a lot of work on your part, but will be well worth it. ANY dog you ever own will need the same precautions to make sure that he stays out of the road and out of trouble. Since you have already let him develop this bad habit, it will be harder than if he was trained from the beginning. You won't regret putting the work into him though.

Think of it this way, he isn't exactly "guarding the flock" if he is chasing semis on the highway is he?